Prostaglandins or prostanoids (PG's) are a group of bioactive compounds derived from membrane phospholipids and are formed from 20-carbon essential fatty acids containing three, four, or five double bonds, and a cyclopentane ring. They fall into several main classes designated by the letters D, E, F, G, H, or I, and are distinguished by substitutions to the cyclopentane ring. The main classes are further subdivided by subscripts 1, 2, or 3, which reflect their fatty acid precursors. Thus, PGI2 has a double ring structure, and the subscript 2 indicates that it is related to arachidonic acid.
PGI2 (also known as prostacyclin) acts on platelets and blood vessels to inhibit aggregation and to cause vasodilation, and is thought to be important for vascular homeostasis. It has been suggested that PGI2 may contribute to the antithrombogenic properties of the intact vascular wall. PGI2 is also thought to be a physiological modulator of vascular tone that functions to oppose the actions of vasoconstrictors. The importance of these vascular actions is emphasized by the participation of PGI2 in the hypotension associated with septic shock. Although prostaglandins do not appear to have direct effects on vascular permeability, PGI2 markedly enhances edema formation and leukocyte infiltration by promoting blood flow in the inflamed region. Therefore, IP receptor antagonists may relieve hypotension related to septic shock, may reduce edema formation, and may prevent conditions associated with excessive bleeding such as, but not limited to, hemophilia and hemorrhaging.
Several in vivo analgesia studies in rodents suggest that PGI2 plays a major role in the induction of hyperalgesia Likewise, in vitro studies provide substantial evidence to suggest that “PGI2-preferring” (IP) receptors act as important modulators of sensory neuron function (K. Bley et al, Trends in Pharmacological Sciences 1998, 19(4):141–147). Since IP receptors in sensory neurons are coupled to activation of both adenylyl cyclase and phospholipase C, and hence, cAMP-dependent protein kinase and protein kinase C, these receptors can exert powerful effects on ion channel activity and thus neurotransmitter release. Evidence of a prominent role for IP receptors in inflammatory pain has been obtained from recent studies in transgenic mice lacking the IP receptor (T. Murata et al., Nature 1997, 388, 678–682).
In addition to being mediators of hyperalgesia, prostaglandins are known to be generated locally in the bladder in response to physiologic stimuli such as stretch of the detrusor smooth muscle, injuries of the vesical mucosa, and nerve stimulation (K. Anderson, Pharmacological Reviews 1993, 45(3), 253–308). PGI2 is the major prostaglandin released from the human bladder. There are suggestions that prostaglandins may be the link between detrusor muscle stretch produced by bladder filling and activation of C-fiber afferents by bladder distension. It has been proposed that prostaglandins may be involved in the pathophysiology of bladder disorders. Therefore, antagonists of prostaglandin IP receptors are expected to be useful in the treatment of such conditions.
Antagonists of IP receptors are also expected to find a utility in respiratory allergies wherein PGI2 production in response to an allergen is present or in respiratory conditions such as asthma.
IP receptor antagonists, as well as a variety of other drugs, have come under increasing scrutiny with regard to cardiovascular side effects. Promising IP receptor antagonists have failed during clinical evaluation because of ventricular arrhythmias, particularly “torsade de points” (TdP), that can occur in clinical trial subjects. The mechanism by which these drugs lead to ventricular arrhythmias has been identified with modulation of the “human ether-a-go-go” or hERG ion channel. The hERG channel is a voltage-activated, inwardly-rectifing potassium channel, and is an important contributor to the repolarization of ventricular action potentials. Blockage of the hERG channel increases the duration of cardiac action potentials, which leads to prolonged ventricular depolarization. Because of the potentially fatal nature of such hERG channel modulation, IP receptor antagonists that will ultimately be useful as drugs should cause minimal or no inhibition of the hERG channel.
Additional information relating to prostaglandins and their receptors is described in Goodman & Gillman's, The Pharmacological Basis of Therapeutics, ninth edition, McGraw-Hill, New York, 1996, Chapter 26, pages 601–616.
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